Word processing packages include several built-in tools that assist writers with vocabulary, formatting, and document organisation, and each tool serves a distinct purpose.
The Thesaurus feature helps a user find synonyms and antonyms of words. When a writer selects a word and opens the Thesaurus, the software suggests alternative words with similar (synonym) or opposite (antonym) meanings, which helps avoid repeating the same word too often and improves the variety and precision of the writing.
The other options describe different, unrelated tools within word processing packages. Checking document statistics (such as word count, character count, or readability scores) is handled by a word count or readability tool, not the Thesaurus. Inserting citations is handled by a references or citation management tool. Creating a table of contents is handled by a dedicated table-of-contents generator that scans heading styles in the document, which again has nothing to do with finding alternative words.
Whenever a question links a word-processing feature specifically to synonyms or antonyms, that feature is the Thesaurus, since none of the other tools deal with word meaning at all.